Field of the Invention
The present invention relates to a process for producing compositions comprising vinylaromatic copolymers which have been produced in a emulsion polymerization process and have production-related salt inclusions, which feature improved surface quality after storage under warm and moist conditions and, in this respect, are suitable for production of shaped bodies having an aging-stable defect-free class A surface. More particularly, the invention relates to a process in which the copolymers are moistened by contact with water vapor or air which has been mixed with water vapor. Moistening is accomplished utilizing a shaft drier, air circulation drier or climate-controlled silo. The granular material pretreated under controlled ambient conditions and for given times is subsequently processed in a compounding step.
Description of Related Art
The present invention additionally relates to the compositions produced by the process according to the invention and to the use thereof for production of shaped bodies compliant with class A surface requirements and having a partial or full high-gloss finish, which may optionally be subjected partly or fully to a further surface treatment step by, for example, painting, in-mold coating of films, metalization via vacuum deposition or electroplating.
Compositions comprising vinylaromatic copolymers prepared by the emulsion polymerization process, and containing production-related salt inclusions, are known from the literature. Sources of such production-related salt inclusions are manifold, for example emulsifier solutions, polymerization initiator solutions, buffer solutions and precipitant solutions which are used as auxiliaries in the emulsion polymerization process, and which, depending on the process, remain in the material or else are removed again from the material only incompletely in the course of workup of the polymer. Especially the precipitation of vinyl polymer latices produced in emulsion polymerization which is generally conducted in conventional processes, as, for example, in EP 459 161 B1, DE 2 021 398 and DE 28 15 098, by means of addition of acids and/or salts makes a considerable contribution to the salt burden of the final polymer, since these salts can generally be removed from the product again by downstream process steps (washing) only insufficiently or with a high level of cost and inconvenience (energy and water/wastewater). Coagulants used are, for example and with preference, aqueous solutions of water-soluble salts, for example alkali metal, alkaline earth metal or aluminum chlorides, alkali metal, alkaline earth metal or aluminum sulfates, alkali metal, alkaline earth metal or aluminum nitrates, alkali metal, alkaline earth metal or aluminum phosphates, alkali metal, alkaline earth metal or aluminum acetates, alkali metal, alkaline earth metal or aluminum formates, alkali metal, alkaline earth metal or aluminum aluminates or alkali metal, alkaline earth metal or aluminum carbonates, more preferably aluminum chloride, calcium chloride and magnesium sulfate solutions, optionally in combination with inorganic or organic acids, for example hydrochloric acid, sulfuric acid, phosphoric acid, boric acid, formic acid, acetic acid, propionic acid and citric acid.
The literature states that such salt inclusions in compositions comprising vinylaromatic copolymers can lead to undesirable effects.
By way of example, WO 2009/071537 discloses that magnesium compounds and/or calcium compounds in impact-modified vinylaromatic copolymers selected from the group of the acrylonitrile-butadiene-styrene copolymers (ABS), acrylonitrile-styrene-acrylate-copolymers (ASA) and methacrylate-acrylonitrile-butadiene-styrene copolymers (MABS), optionally comprising polycarbonate and additives, lead to undesirable deposit formation on the shaping mold in the course of thermoplastic shaping via injection molding or extrusion, and in that respect claims compositions of this type having a content of magnesium compounds and/or calcium compounds of 0 mg/kg to 100 mg/kg. The emulsion polymers used in said compositions are usually precipitated by freeze precipitation in a flake ice machine, rather than by the conventional addition of magnesium sulfate solution.
WO 98/28344 discloses a process for continuous coagulation of aqueous dispersions of graft rubbers via shear, which overcomes the known disadvantage of precipitation by means of acids and/or salts as coagulants, in that contaminants often remain in the polymers after workup, and these can lead to impairment of product properties.
One problem with thermoplastic compositions comprising vinylaromatic copolymers prepared by the emulsion polymerization process with production-related salt inclusions is that when moldings produced from these are exposed to moisture (for example condensation or humidity), particularly at elevated temperatures, they are susceptible to undesirable development of surface defects (blistering), which restrict the use of compositions of this type in moldings with a high-gloss finish and those compliant with Class A surface requirements.
EP 2 398 842 A1 discloses a compounding process for production of impact-modified polycarbonate compositions having a reduced content of volatile organic compounds, in which 2% to 40% by weight, based on the sum total of impact modifier and water, of liquid water is added to the pulverulent graft polymer used as impact modifier and the preliminary mixture thus prepared is used in the compounding of the impact-modified polycarbonate compositions. This process corresponds to a comparative example in this application.
DE 102 009 007 789 describes a standard shaft drier. A unit of this kind is used to dry bulk goods.
WO0143556 describes the moistening and ozone treatment of flour in a moistening silo. There is no mention here of polymer granules.
JP11056092 discloses a rice silo having moisture regulation (including spray introduction nozzles) for applications in the food industry. There is likewise no mention here of polymer granules.
WO9857110 describes a silo for controlling moisture content. It is possible here to intensify drying of the air. Active moistening of the air is impossible.
US2007256316 discloses a shaft drier for soy beans, in which the soy beans are purely dried. Moistening is likewise not envisaged.
DE3303164 describes the production of non-sintered fine iron ore material with cement as hydraulic binder. Pellets are present in the shaft reactor. There is a pretreatment zone with relative humidity<=70%, a hydration zone with introduction of saturated steam at 50° C. to 100° C., and a drying zone. Condensation in the hydration zone is desirable and is used to heat the granules.